Hydraulic transmission



w. FERms HYDRAULIC TRANSMISSION Filed Dao. 21. y192s s sheets-sheet 1 YmVENToR.

` mn TER FMH/5 ATTORNEY.

Oct. Z0, 1925.

w. FERRIS.

HYDRAULIC TRANSMISSION med nec. 21, 1925 3 Sheets-Sheet 2 BY 'WurmFERR/s A TTORNEY.

W. FERRIS HYDRAULIC TRANSMISSION Filed Dag 21, 1923 s sheets-sheet s FfIMMER Feng/5 ATTORNEY.

' Patented Oci. 20,4 1925.A

' UNIT-Eo s-'rA'rss PATENT oFFici-z.'

WALTER FERRIS, MILWAUKEE, WISCONSIN, ASSIGNOR T THE OILGEAR COMPANY 0FMILWAUKEE, WISCONSIN, A CORPQRATION 0F WISCONSIN.

HYDRAULIC TRANSMISSION.

Application filed December 21, 1928. SerialNo. 682,056.

To all whom it may concern.'

' Be it known that I, WALTER FRRIS, a

citizen of the United States, residing at Milwaukee, in the county ofMilwaukee and State of Wisconsin, haveinvented a certain new and usefulImprovement inHydraulic Transmissions, of which the following is a full,clear, concise, and exact description, reference being had to theaccompanying drawings, forming a part of this specification.

'This invention relates to hydraulic'transmission. l*

. p The general aim of the present invention, 1s to improve theconstruction and operation of `devices of this character.

' A more specific object is to improve the construction and arrangement,of the pum parts' and their connections with the hydraulic circuitthereby to obtain a more convenient control of the -rate and directionof fluid flow in the circuit.

Another object is the provision of improved means for taking care ofdifferences in the volume of uid in the two sides ofthe circuit, whichordinarily occur, for instance, in those systems in which a double' Inthose instances in which the motor constitutes a verticallyarrangedpiston and cyl- `inder assembly there `seems to be a tendency for thepiston to creep downwardly by gravity.. 'Another object of the presentinvention is the p rovision of a simple means for avoiding thisdifficulty. j

Other objects and advantages will appear from the following descriptionof an illus- 'trative embodiment of this invention.

application, Serial No. 483,468, tiled July and as an integral Apart ofthe Casin 10.

In the drawings: Figure lisa sectional view through the casing of a pumpcomprising a part of a hydraulic transmission system embodying featuresof the present invention.

Figure 2 is a sectional view taken substantially along the line 2-2 ofFigure 1.

Figure 3 is a view illustrating the circuit connections between the pumpdischarge and a vertical powercylinder of a typical hydraulic press.

Figure 4 is a -fragmentary sectionall view of the pump shown in sectionin Figure 1. In the hydraulic transmission shown, the pump andassociated parts are enclosed in a casing 10 having a basin or sump 11in the base thereof containing a reserve supply of working fluid,preferably oil. `A- drive shaft 12, entering the casing 10,is keyed atits inner end 13 to a hub 14 of the pump driving cage 15, and, at itsouter end car- 'ries a heavy lywheel-'drivin pulley 16. This shaftisjournaled in a Xed bearing 17 in the wall of a housing 18 fixed to theoutside of the casing 10, i

The pump shown is similar in many respects to that described in mycopending 9, 1921. In this instance, however, the cage 15 rotates abouta fixed axis and the pintle 19 is adjustable to vary pump displacement.The cage is journaled at one side in a fixed bearing 20 provided in aremovable plate 21 f ormng one part of the casing wall, and at the otherside in fixed bearings 22 provided in a rigid structure 23 formed withinThecage 15 l'carries adjacent its perip ery a series of tan er1-tial`reaction plates 24, removably fixe in opposed grooves 25 formed in theperipheral walls 26 of the cage. A cylinder barrel 27 having a radialseries of cylinder bores 28 is supported by and rotatesupon the pintle19. Each cylinder bore has a port 29 adapted to register successivelywith ports V3() and 31 formed in the upper and lower surfaces of thepintle 19, during rotationof the cylinaction plate within limits definedby a resages taining pinik 37 anchoredin the reaction plate and looselyengaging a slot 38 in the roller cage.

The pintle 19 is fixed in and carried by an upwardly projecting arm 39having a hub 40 rockahly mounted upon a shaft 41 extending across thecasing 10, and having one end 42 fixed in one wall of the casing and theother end fixed in the rigid structure 23 hereinabove mentioned. Upperand lower pairs of passages 43 and 44 in the pintle communicate with thepintle ports 30 and 31, respectively, and with passages 45 and 46,respectively, leading downwardly along the arm 39 to passages oppositesides-of the hub portion 40. Pas- 47 and 48 communicate ,at all timesthrough ports 49 and 50 with two vertical pairs of passages 51 and 52,respectively, extending longitudinally of the .shaft 41. lassages 51 ofVone pair communicate with a port 53 formed in the end 42 of shaft 41and passages 52 of the other pair communicate with a similar port 54.

A cap 55 covering the projecting end 42 or" shaft 41is fashioned toreceive appro- Y priate pipe connections through which the pump may beconnected with any form of hydraulic motor desired. (See Figure In thisinstance, the motor shown is a vertical power cylinder 56 of a typicalhydraulic press. A pipe 57 is shown connecting the upper end ot'this'cylinder with the `port 54 while the other port 53 is shownconnected Awith the lower end of the cylinder through pipes 58 and 59and a valve mechanism to be hereinafter described. v

`The arrangement of parts thus far described is Such that when thepintle 19 is in a neutral position in which the axis of rotation of thecylinder barrel 27 is coincident with the axis of rotation of thedriving cage 15, rotation imparted to the driving cage 15 by the pulley16 and shaft 12 will cause the pistons 32 to travel about the pintle 19without reciprocating in the coi either operating cylinder bores 28 andpump displacement is zero. Any slight rocking of the arm 39 about theshaft 41 so as to shift the pintle 19 substantially horizontally indirection from this neutral osition will cause the pistons toreciprocate' 1n their cylinder bores during rotation of the drivthepiston 60 therein At the same time, iiuid discharged trom the- 47 and 48inV ing cage. When the pintle 19 is thus shifted in one direction to theeccentric position shown, for instance, in Figure 4, rotation O the cage15 in a clockwise direction causes the pistons, during the upper half oftheir circular travel, to drive fluid from their cylinder bores throughthe ports 29 and pintle port 30'to and through the upper pair otpassages 43 in the pintle. The fluid thus delivered by the pump passesdownwardly through the passages 45 and47 in the arm 39, through thepassages 51 in the shaft 41, and thence through pipes 58 and 59 to thelower end of the power cylinder 56 causing to travel upwardly.

upper end of this cylinder passes through the pipe 57, passages 52, 48,46 and the lower passages 44 in the pintle from which it is returned tothe cylinder bores in the lower half of their circular travel throughthe pintle port 31. Similarly, pintle to the other side'ot its neutralposition, the cylinder bores during the upper half' of their circulartravel receive iiuid from the npper'pintle passages 43 and during thelower halt of their travel discharge iiuid into and through the lowerpintle passages 44. ylhus, by swinging the arm 39 the pintle may heshifted so as to vary the direction and rate ot iiuid How delivered bythe pump.-

The position of the arm 39 and pintle 19 may be varied and controlled byany appropriate means. 1n this instance, the arm is provided with a pairot upstanding spaced ears 61 operatively connected through a by shittingthe ipo pair of links 62 with a hollow plunger 63,

mounted for lengthwise shifting in a control cylinder 64, secured withina wall ot the casing 10. A piston 65 reciprocable in the cylinder 64 isformed integral with or with the interior of the hollow plunger 63,'

the plunger having a port 76 open at all times to the interior of thepump casing 10. A second longitudinal duct 77 in the valve communicateswith the two annular chambers 71 and 73 through ports 78 and 79,respectively. An annular chamber 8O formed in the inner wall of a sleeveportion 81 of the control cylinder 64 .communicates times with thechamber 73 through a port 82 and is supplied with iuid under pressurethrough a passage 83 from a convenient pressure source, such, forinstance, as will be hereinafter described. .Sets of ports 84` at alland 85 through the wall of the plunger 63 positionshown in Figurel 2, sothat ports 84 and 85 'are uncovered by heads 68 and 69, luid'underpressure passes from the chamber 80, 4through port 82,`ch'amber 73, andports 85 into the right end of control cylinder 64. Simultaneously, theleft end of control cylinder is opened to exhaust through the ports 8.4,chamber 72, port 75, duct 74 and port 76. This, of course, re-

sultsin movement of `the piston 65 and plunger 63 to the left until theports 84 and 85 are again covered by the heads 68 and 69.

. The extent of movement of the piston and plunger is, of course,dependent upon the extent to which the pilot valve has been shifted,this movement of the plunger being transmittedto the arm 39, and pintle19, through the links 62 and ears 61. (Figure 1.) When the pilot valve66"'is shifted to the right, fiuid under pressure is delivered to the.left end of'control c linder 64 through'the ports 84, chamber 7,1,

port 78,-duct '77, port 79, chamber 73 and port 82 'from chamber 80,andthe right end of cylinder 64 is openedto exhaust through ports 85,chamber 72 and duct 74. This results in movement of the piston'65 andplunger 63 to theright a distance' corresponding to the extent ofmovement of the pilot valve. The pilot valvev 66 may be shifted and-controlled by any appropriate mea-ns as typilied by the hand wheel 86onA a shaft 87 having a pinion and rack eonnec tion 88 with the stem 89of the valve.

It will be noted that a variable displacement pump has been providedAhaving the desired flexibility without. requiring shifting of the driveshaft 12 or the use -of ilexible pipe connections.

During operation of the machine shown, iuid under substantially constantpressure is maintained in the chamber 80 by mechanism now to bedescribed. Referring to Figure 1, this mechanism includes a gear pump 90of a well known construction mounted in a casin`g 91 secured tothestructure 23 adjacentthe end ofv shaft 41. This pump is driven by ashaft 92, carrying a vdriving sprocket 93, connected through anappropriate chain 94 with a sprocket 95, keyed' to the pump drive shaft12. Fluid from the base-of the sump 11 is lifted by the pump through apipe 96 into the pump casing and is discharged by the pump through apipe 97 into a passage 98 passing through a wall of the main pump casing10. Passage 98 communicates with pasdegree of fluid sage 83 leading tothe" chamber 80. A

Aspring loaded relief valve determines the pressure existing in thepassage 83 and chamber 80. This valve comprises a ball 99 normallyseated over a port desired tension in the spring. The quantity of iuiddelivered to the passage 83 is preferably always in excess of thatdemanded by the control mechanism, 'the excess being discharged -pastthe ball valve 9 9 and through the port 104.

In order to protect the hydraulic transmissionV against excessivepressures advantage is taken of the fact that in a pump ofA thetypedescribed, the pintle tends to shift toward'its intermediate or zeropump stroke position by reason of an unbalanced thrust thereon resultingfrom the eccentricity of the cage 15 and cylinder barrel 27. It has beenfound that this thrust is substantially proportional to the workingpressure in the I tion,- the piston 65 is selected, of such diameter,that with a iven fiuid pressure in thechamber 80, the piston isv capableof producing a thrust upon the .plunger 63suH cient to shift and retainthe pintle 19 at any desired position of veccentricity, so long as thepressure., inthe main hydraulic 4circuit is below a desired maximum..Assuming, for instance, that the transmission is operating atv apressure below this maximum,

with the pintle 19 in a position offset to the left of the zero pumpstroke position, so that the pump thrust upon the pintle tends to strokeposition. The pintle is retained in the right endof the cylinder 64(Figure 2), the plunger 63 and pilot valve 66 being in the relatlvepositions shownwith the ports 84 and 85 covered by the heads 68 and 69on the pilot valve. Now, should the pressure in the main hydrauliccircuit increase over the allowed maximum, the pump thrust upon thepintle increases accordingly-until this thrust. transmitted to theplunger 63 and piston 65 is suiiicient to force leakage of fluid fromthe right end of cylinder 64 and past thehead 69 against the constantpressure of fluid maintained in the chamber 73 from they chamber 80.This leakage of fluid permits the piston 65 to move to the right,openin'f ports 85, and thus permitting further movement thereof andconsequently the pintle to the right to reduce the pump stroke.Reduction of the pump Stroke'thus continues u'ntil the pressure in thehydraulic system has been reduced below the allowed lhydraulic circuit.In utilizing this condiloo Y force the pintle to the right toward thezero chamber 80 against the fiuid pressure therein. Since the maincircuit pressure at which this action will occur is dependent upon thepressure maintained in the chamber 80, this maximum pressure may bepredetermined by the adjusted tensionin the spring 102 of the reliefvalve. For purposes ot convenience, a pressure gauge 105 may beconnected with the passage 83 to indicate the pressure therein and inthe chamber 80.

Considerable diiculty has been experience-d in attempting to shift thepintle into zero stroke position with the extreme accuracy required toreduce the iiow in the main hydraulic circuit to absolute zero.

' Anydeviation from this theoretically correct position results inundesirable creeping of the motor due to the slight flow in the circuit.1n order to avoid this difficulty, provision is made 'for automaticallyshort circuiting the two sides of the circuit as the pintle assumes zerostroke position. In this instance, the mechanism orthis purpose includesa lever 106', depending from a piv'ot pin 107 fixed in appropriate lugs108, formed uponthe top-wall of the main casing 10. Adjacent its upperpivoted end this lever is formed with an open ended horizontal cross-cut109. A pair of plates 110, one at each side of the lever, cover the openends of the cross-cut 109. These plates are carried'by a pair of tensionrods 111 exten-ding loosely through the lever above and below thecross-cut. ICoiled compression springs 112 on the rod 111 yieldablyretain the plates 110 in contact with the sides of the lever. One of theears 61 on the pintle supporting arm 39 carries an integral lug 113,projecting loosely into the cross-cut 109, between and in contact withthe plates 110, so that as the arm 39 is swung in either direction fromthe neutral position shown in Figure 2, the lug 113, acting upon one orthe other plates 110, tends to shift the lever 106 in the samedirection.

The lower free end 114`of1the lever 106 is connected to the stem 115 ofa short circuiting valve 116v reciprocable in a valve block 117 securedto a rail 117 in the 'casing. The V valve with three annular chambers118, 119 and 120. Passages 1 21 and 122 connect the chambers 118 and120, respectively, with the passages 51 and 52 constituting parts of thetwo sides of the main hydraulic circuit.

block shown is providedv The valve 116 is provided withan intermediatereduced portion, 123 permitting communication between the chambers 118and 120 when the valve is in the intermediate position of Figure 2. Withthe valve in this position, the two sides of the main circuit are inopencommunication through the valve block. This is the position of the partswhen the pintle is in zero pump stroke osition.

When the arm 39 and pintle are shifted in either direction from thisintermediate position, the lug 113, acting upon one of the plates 110,Causes the lever 106 to swing about its pivot 107 Iuntil the lower endthereof has shifted the valve 116 into such position as to closecommunication between the chambers 118 and 120 in the valve block.Further movement of, the valve 116 is prevented by the engagement or"one of the stop collars 124 or 125. carried thereby, with the outersurface of the valve block. Since the lug 113 is so close to the pivot107 of the lever as compared with the length thereof, this closingmovement of the valve 116 is eli'ected by a very slight movement of thepintle supporting arm. Further movement of the pintle may be continued,without movement of` the lever 106, since either plate 110 is permittedto yield away from the lever against the pressure of the springs 112. it.will thus be understood that the pintle may be shifted in eitherdirection and to any desired extent to obtain the desired pump stroke,and during its initial movement the valve 116 is suddenly shifted so asto'cut oli communication of the twosides of the circuit through thevalve block 117.

To insure against building up of pressure in the circuit during theshort circuiting position of the valve 116, a discharge duct 126 isprovided in communication 1with the intermediate chamber 119 in thevalve block 117. This duct is controlled by a check valve 127 of anyappropriate type.

Mechanism for taking care of the difference in volume of fluid in thetwo sides of the main hydraulic. circuit will now be de-. scribed. Suchmechanism has particular utility when, as in this instance, thehydraulic motor is in the form of a double acting cylinder. lForinstance, as the piston 60 travels downwardly in the cylinder 56 agreater amount of Huid must be fed into the upper end of the cylinderthan is discharged from the lower end thereof to compensate for thedisplacement of the piston rod 60 in the lower end of the cylinder only.In the pump shown, the mechanism for taking care ot this uid diterentialcomprises a valve block 129 having a cylindrical bore 130 therein formedwith an intermediate annular chamber 131. A piston valve having threepiston heads 132, 133 and 134: and reciprocable in the block, dividesthe lll) .136, 137 and 138.

bore 13() thereof into four chambers 135, The valve is shown in Figure'l in intermediate position in which, the intermediate head-133closes communication between the chamber 131 and the rest of the bore130. Chambers 136 and 137 are always in communication .with the passages45 and 46, respectively, through passages 139 and 140, and chambers-135and 138 are always in communication with chambers 137 and 136,respectively, through passages 141 and 142. A passage 143 leads from thechamber 131 to the sump 11, preferably through'a two way check valve14,4 to be later described. The arrangement Vis such that when thepassage 46 is carrying the working fluid, the pressure of this lluid istransmitted 'through passages 140 and 141 to chamber 135 causing thepiston valve to shift to the right (Figure 2), thereby open-` ingcommunication between the return side of the circuit and the sumpthrough the pas sage 139, chamber 136, chamber 131 and the passage 143.This is the position of the piston valve when fluid is being deliveredtov the upper end of cylinder 56 causing the pis-V t0n 601:0 traveldownwardly. As above pointed out, the upper end of the cylinder' D56demands a greater volume of fluid than that discharged by the lower endthereof. This deficiency of fluid volume is made up by fluid sucked upby the main pump into the passage 45' from the sump through the passage143, chambers 131 and 136 and passage 139. When the displacement of the.main pump is reversed so that the piston 6() is traveling upwardly, andpassage 45 is carrying the'workin'g fluid and passage 46 thereturnfluid, the greater pressure in passage` is transmitted through thepassages 139 and 142 to the chamber 138, to therbv shift the pistonvalve to the left, and thereby open communication between chambers 137and 131. With the piston valve in this `position, communication isestablished between what is then the return passage 46 and the sump,through the passages 140, chambers 137 and 131, and the passage 143. Theexcess luid discharged by the upper end of the cylinder 56 is thusreturned to the sump. In order to insure 'sulicient pressure in thechamber 135 or 138 toeifect reversal of' -the piston valve, when thepump is reversed,

check valve 144 is preferably etween the passage 143 and the a two waprovided sump. The checl': valveV shown comprises a A valvecasing 145communicating with the pipe or passage 143 and with the 'sump 11 lthrough a duct 146. A tubular valve 147 is yieldably maintained againstits seat 148 under the pressure of a light spring 149, and adapted toopen under the pressure of Huid passing downwardly through the passage143. A tubular valve 147 is. rovldedwith'a port 150 normally covered y aball valver151, arranged to be lifted to open po.v i

sition by flow of fluid upwardly into the passage 143. It will beunderstood that the return side of the circuit, which is in opencommunication with the passage 143,.'becomes the working side of thecircuit when the pump is reversed, and were it not for the check. valve144, the fluid in this side of the circuit would be free to escape intothe sump without actuating the piston valve. The checkvalveserves to socheck the flow of fluid in the passage 143 as to provide,

sutlicient pressure in the work-ing side of the circuit to shift thepiston valve when the pump is reversed.`

Provision is preferably made for preventing the piston 60 from creepingdownwardly by gravity, as, for instance, when the motor circuitV isshort-circuited through the block 117 or otherwise. This is accomplishedin this instance by the use of a two 'way check valve interposed betweenthe pipes 58 and 59. This valve comprises a casing-152 -having ports 153and 154 in communication with pipes 58 and 59, respectively. A hol-l lowvalve 155 within the casing is yieldably retained in closed position bya spring 156. This valve is provided with a port 157 con- .trolled by aball check valve 158, arranged to open freely by the flow of fluid frompipe 59 .to pipe 58, and to prevent flow through v the hollow valve 155in the other direction. The tension in spring 156 is such as to retainthe hollow valve on its seat against the pressure of fluid in the pipe58 due to the weight of piston 60, but to permit this valve to move fromits'seat under the fluid pres-- sure in the pipe 58 during normaloperation of the'system. Thus, as the piston 60 is' traveling downwardlyunder the working pressure in pipe 57, the fluid from the lower end ofthe cylinder passes from pipe 58 through port 153, around the valve 155,and through port 154 into pipe 59.

Various changes may e made in the em-l 'bodiment of the inventionhereinabove described, without departing from or sacrifie-- ing any ofthe vadvantages of the inventionA as defined in the appended claims.

I claim:

1. In a hydraulic transmission the combination of a hydraulic motor, astationary delivered from said pump,

and motor comprising a stationary member ,1.30

j municating with said pump,

and a second member rockably associated with said stationary member andoperable to vary and control the relative displacements of said pump andmotor.

3. ina hydraulic transmission the combination oa -variable displacementpump, a stationary member havin a fluid passage therein, and a secondmem er rockably associated with said stationary member Jfor regulatingpump displacement, said second member having a tluid passage thereincommunicating with said pump andwith said rst named passage.

4. ln a hydraulic transmission the combination ofY a variabledisplacement pump, a rockable member for regulating pump displacementhaving `a Huid passagecomand a pivotal support for said member having aiuid said first passage communicating with named passage 1n allpositions of said member.

5. lin a hydraulic transmission the combinationY of a variabledisplacement pump, a roekable member for regulating pumpdisplacementchaving separate passages for receiving fluid from anddelivering Huid to said pump, and a pivotal support for said memberhaving separate passages communieating respectively With said firstnamed passages in all positions of said member.

6. In a hydraulic transmission the combination of a variabledisplacement pump having a pintle adjustable with changes in pumpdisplacement, said pintle having a passage through which tluid passes toor from saidpump, a shaft having a fluid passage, and a link connectingsaid pintle and shaft and maintaining communication between saidpassages in all positions of said pintle.

7. A variable displacement pump or motor having a pintle adjustable withchanges in pump displacement, and a support for said pintle roekableabout 'an axis oi'set from the axis of said pintle.

8. ln a hydraulic machine the combination of a radial series of istonand cylinder assemblies, a pintle aving passages communicatingtherewith, and a pintle support having passages communicating With saidfirst named passages andv rockable about an axis otset from the axis. ofsaid pintle to vary the position of saidpintle relative to saidassemblies.

9. n a hydraulic machine the combination of a radial series of pistonand cylinder assemblies, a pintle substantially centrally disposedwithin said series, and a support for said pintle rockable about an axisoffset therefrom to vary the 4position of said pintle relative to saidassemblies.

10. In a hydraulic machine the combination of a pintle, a seriesoipiston and cylinder assemblies grouped about said pintle,

an adjustable support for said pintle, and hydraulic means, for.adjusting said support to vary the Working stroke of said assemblies.

11. In a hydraulic machine the combinato thereby vary the center ofrotation o said series of assemblies.

12. In a hydraulic machine the combination of a pintle, a series ofpiston and cylinder assemblies .rotatable about said pintle, anadjustable support for said pintle, a source ofiuid pressure, and meansactuated from` said pressure source for adjusting said support.

ln a hydraulic machine the. combination of a pintle, an adjustablesupport therefor, a series of piston and cylinder assemblies rotatableabout said pintle, a pilot valve, and hydraulic means controlled by saidvalve for adjusting said support a distance corresponding to the extentof movement of said yalve.

14.1 n a hydraulic transmission the combination of aA variabledisplacement pump, a hydraulic circuit fed thereby, hydraulic means forregulating pump displacement, said means being of such capacity as toyield under pump thrust to reduce pump displacement when\ the pressurein said circuit exceeds a l predetermined maximum.

15, ln a hydraulic transmission the combination of avvariabledisplacement pump,

a hydraulic circuit fed thereby, a source of substantially constant Huidpressure, and a piston actuated from said pressure source Jforregulating .pump displacement and yieldable When the pressure in saidcircuit exceeds a predetermined maximum to reduce pump displacement.

16. ln a hydraulic tranmission the combination of a variabledisplacement pump, a hydraulic circuit fed thereby, a pilot valve, and afluid actuated piston for varying pump displacement in accordance withthe extent of movement of said valve and automatically operable toreduce pump displacement When the pressure in said circuit exceeds apredetermined maximum.

17. ln a hydraulic transmission the combination of a variabledisplacement pump, l

control means for regulating pump displacement, a hydraulic circuit fedby said pump, a by-pass `connecting the sides of said circuit when saidcontrol means is substantially l1n zero displacement pos1t1on, and meansactuated only by the initial movement of said control means from zerostroke position for closing said by-pass.

18. In a hydraulic transmission the combination of a pump having anadjustable control means therefor, a hydraulic circuit and said controlmeans. v Y

20. In a hydraullc transmission the comtherefor,

fed by said pump, short-circuiting means for said circuit, and meansactuated by said element -for controlling said short-circuiting means.

19. Ina hydraulic transmission the combination of a variabledisplacement pump, control means therefor, a hydraulic circuit fed'bysaid pump, short-circuiting means,

actuating means therefor', and -yieldable connections between saidactuating means binationof a variable displacementpump,

control means therefor, a hydraulie'circuit fed by said pump, ashort-circuitlng valve for said circuit, and yieldable connectionsbetween said control lmeans and said valve for actuating said valve. q

,21. In a hydraulic transmission the combination of a variabledisplacement pump,

Vcontrol means therefor, a hydraulic circult fed by said pump, ashort-circuiting valve for said circuit, alever for actuating saidvalve, and connections between said lever and said control means.

22. In a hydraulic transmission the combination-of a variabledisplacement pump,

control means therefor, a hydraulic circult `fed by said pump, ashort-circuiting valve for said circuit, a lever, a pivotal support.connections between lsaid control means and said lever relatively nearsaid support, and connections between said .valve 'and said leverrelatively far from said support. i

23. In a hydraulic transmission the combination of a variabledisplacement pump,

control means therefor, a hydraulic circuit,

a short-eircuitiug valve, a lever, connections betweensaid lever andsaid'valve,jyieldable connections between said lever and said controlmeans, and a support for said 'lever pivotally connected therewithl at avpoint relatively near said yieldable connection and far from said firstnamed connection.

a hydraulic circuit fed thereby, means for short cireuiting said circuitwhen pump displacementis zero, and means'for destroying the pressure insaid circuit when said short a Fluid reservoir, and meansautomatically.'

operable to elfect and maintain communica- -tion between saidreservoiriand that side of said circuit containing low pressure uid.

26. In a hydraulic transmission the combination of a reversible flowpump, ahydraulicV circuit communicating therewith Vhavinginterchangeable highand low pressure sides of relatively varyingcapacities, .y

-and means for yieldably resisting the flow of'iiuid through said meansin one direction.

27. In a. hydraulic transmission the combination with a hydrauliccircuit including a cylinder, a reversible flow pump, and Huidconnections between saidpump and the opposite ends of said cylinder, ofa piston in said cylinder',` and means including a valve automaticallyoperable to compensate for relative variations in the volumetriccapacities of the two sides of said circuit.

28. In a hydraulic transmission the combinationof a hydraulic circuithaving interchangeable high and low pressure sides of relatively varyingcapacities, a Huid reservoir, a valve operated by the high pressurefluid in one side of sa-id circuit for opening communication betweensaid reservoir and the other side of said circuit, and a check valveassociated with said rst named valve.

29. In a hydraulic transmission the combipassages, whereby a greaterpressure in one side of said circuit will shift said valve to opencommunication between the other side of said circuit and said port.

30. Ina hydraulic transmission the combi- A nation of a pump, ahydraulic motor, a hy- 24.- Iii a hydraulictransmission the combinationof a variable displacement pump,-

draulic circuit connecting @said pump and motor, and means for yieldablyresisting the' flow of iiuid in one direction in said circuit to preventcreeping of said motor when said pump is ineffective.

`31. In a hydraulic transmission the combination of a cylinder, a pistontherein, a hydraulic circuit connected with said cylinder for operatingsaid piston, and means for yieldably yresisting a flow of fluid from oneend of said cylinder to prevent undue creeping of said piston when saidcircuit is idle.

' 32. In a hydraulic transmission the combination of a substantiallyvertical cylinder, a piston therein, a hydraulic-circuit connected withsaid cylinder for actuating said piston, and means for yieldablyresisting a `iiow of fluid from the lower end of said nation of ahydraulic circuit having intercylinder to prevent said piston fromcreeping by gravity therein.

33. In a hydraulic transmission the combination of a reversible pump, aclosed hydraulic circuit communicating therewith having interchangeablehigh and low pressure sides of relatively varying capacities, a Huidreservoir, 'and means responsive to the diti'erence in pressures betweenthe sides of said circuit-for maintaining communication between saidreservoir and the low pressure side of said circuit. y

34. In a hydraulic' transmission the combination of a reversible owpump, a' closed hydraulic circuit having sides `of relatively varyingcapacities connected to the opposite sides o said pump, a iuidreservoir, and valve mechanism responsive to the dierence in pressuresbetween the sides of said circuit for connecting said reservoir withthat side only of said circuit containing low pressure fluid.

35. In a. hydraulic transmission the combination of a reversible flowpump having interchangeable intake and delivery passages, a. closed'hydraulic circuit having sides of relatively varyin'g capacitiesconnected to said pump passages, respectively, a fluid reservoir, and avalve connected across the sides of said circuit and automaticallyoperable to maintain communication between said reservoir and the intakepassage of said pump.

in Witness whereof, hereunto subscribe my name this 5th day of November,1923.

\ WALTER FERRIS.

